Beyond Two-State Conical Intersections. Three-State Conical Intersections in Low Symmetry Molecules:  the Allyl Radical

Using multireference configuration interaction expansions comprised of over 7 million configuration state functions, three-state conical intersections are reported for the closely spaced, spectroscopically observed B̃(2A1), C̃(2B1), and D̃(2B2) states (in C 2 v symmetry) of the allyl radical. These conical intersections of states which were previously assigned as the 3,4,52A states and are here reassigned as the 4,5,62A states, are expected to be accessible using optical probes. This conclusion is obtained from the structure of the minimum energy point on the 4,5,62A three-state conical intersection seam which is similar to the equilibrium structure of the ground X̃(2A2) state and only 1.1 eV above the D̃(2B2) state at its equilibrium geometry. The seam of three-state degeneracies joins two two-state seams of conical intersection, the 4,52A and 5,62A conical intersection seams. The energy of the minimum energy point on the 4,52A two-state seam is only 0.15 eV above that of the D̃(2B2) state at its equilibrium structure. Three-state intersections are also reported for the 3,4,52A states.